1. Field of the Invention
The present invention relates to control apparatus for providing a rotary motion and, in particular, to apparatus for imparting rotary control motion to rotary valves.
2. Description of the Invention Background
Various methods and devices for imparting rotary motion to rotary actuated devices are known in the process control industry. For example, ball and butterfly valves that are used for controlling the flow of a myriad of liquid materials ranging from milk to unrefined oil are commonly controlled by such actuators. In addition, rotary actuators have also been used to impart rotary motion to indexing tables on small part assembly lines in the electronics industry. In general, rotary actuators have also been used in a plethora of other process and assembly operations requiring rotary motion.
Typical rotary actuation devices, such as those disclosed in U.S. Pat. Nos. 3,602,478, 3,985,151 and 4,325,535, generally consist of an airtight housing having a flexible diaphragm fixed therein. A pressure medium is introduced into the housing to thereby cause the diaphragm to flex and impart a linear motion. This linear motion is thereafter converted to rotational motion through the use of conventional ballscrews, linkages or splined shaft connections. Such devices are usually large and cumbersome and generally require more upkeep and replacement parts than the devices they are designed to control.
The prior designs have disadvantages in several respects. In the previous diaphragm operated designs, the diaphragm is fixed to the airtight housing by a series of fasteners that extend around the diaphragm's perimeter. Constant cycling and flexure of the diaphragm causes localized stresses to build up in the diaphragm material around the points of attachment. Continued cycling of the diaphragm eventually causes cracks or ruptures to develop in the diaphragm material. Such ruptures permit the pressure medium to pass through the diaphragm without causing the diaphragm to impart the requisite linear motion thereby rendering the actuator inoperable. To replace the diaphragm, a large number of retaining bolts usually must be removed. This operation is time consuming and requires the use of various hand tools. Often, the actuator is mounted in a cramped area that is not conducive to the manipulation of hand tools which can complicate the diaphragm's removal and thereby lengthen the repair time.
In most prior devices, the diaphragm is enclosed within an airtight metal chamber that affords no visual access for monitoring the diaphragm's integrity. Therefore, in devices of this type, actuator failure cannot be prevented by routine maintenance inspections unless the actuator is taken out of service and disassembled.
In another form of prior art remote control valve operators, such as shown in U.S. Pat. Nos. 2,998,805 or 4,647,003, the valve operator includes a chamber having a movable piston therein and means for applying fluid pressure on each side of the piston to cause its linear displacement. The piston is directly attached to mechanical means for translating the linear displacement of the piston into rotary motion.
I have become aware that such forms of valve operators are also fraught with serious shortcomings. In particular, both the machining of the inner wall of the cylinder and the outer edge of the piston must be exceptionally accurate to avoid binding of the piston. Also, rotational forces on the piston may cause it to rotate slightly to cause excessive wear on the piston seal or on the cylinder wall. In addition, the mechanical components connected to the piston create forces thereon which prevent perfectly concentric movement of the piston in the cylinder.
The present invention is directed toward an improved design for actuating devices having rotary control means which overcomes, among others, the above discussed problems and provides for a more reliable and maintainable means of rotary actuation.